Sulfonamide polymers are incorporated into a variety of materials and devices. One of the major areas where sulfonamide polymers have been proposed for use is in the field of separations. For example, sulfonamide polymers can be used to prepare semi-permeable membranes, which have been proposed for use in separating solution components. Such membranes are a type of filter able to retain certain substances while transmitting others. The components of the feed fluid that pass through the membrane comprise the “permeate” and those that do not pass through the membrane (i.e., are rejected by the membrane or are held by the membrane) comprise the “retentate”. In practice, the permeate, the retentate, or both streams may represent the desired product and may be used as obtained or may be subjected to further processing. In order to be economically viable, the membrane must provide sufficient flux (the rate of permeate flow per unit of membrane area) and separation (the ability of the membrane to retain certain components while transmitting others).
Typically, polysulfonamide membranes are made of polymeric materials having a polymer backbone comprising a plurality of primary sulfonamide groups (—SO2—NH—). Based on their chemical structure, polysulfonamide membranes might be expected to be stable in the presence of concentrated bases. However, it has been found that even relatively short exposure to elevated pH (e.g. about 10), can cause a polysulfonamide membrane to lose performance.
Primary sulfonamide polymers (polymer-SO2—NH-polymer) have an acidic proton on a sulfonamide nitrogen. When exposed to sufficiently basic solutions this proton is removed and the sulfonamide group becomes negatively charged (as shown in the following scheme).polymer-SO2—NH-polymer+OH−→polymer-SO2—N−-polymer  (2)
This deprotonation renders many polymeric sulfonamides swellable. Such swelling often results in a polymer (e.g. a membrane polymer or film) that is mechanically weak and highly susceptible to damage.
Secondary sulfonamide polymers (polymer-SO2—NR-polymer, wherein R is not H), which have been formed, for example, by the reaction of an activated sulfonyl compound and a secondary amine (see for example, R. C. Evers and G. F. L. Ehlers, J. Polymer Science, 1967, 5, 1797–1801), are incapable of being deprotonated at the sulfonamide nitrogen by base. As a result, such polymers may not swell when exposed to base.
This stability makes secondary sulfonamide polymers desirable materials, particularly for use in carrying out separations at high pH. Unfortunately, the number of secondary sulfonamide polymers available for carrying out separations is severely limited by the relatively slow reactivity of many secondary amines under the conditions typically used for preparing sulfonamide polymers.
Currently, the lack of base stability limits the use of primary sulfonamide polymers. Additionally, the unavailability of secondary sulfonamide polymers, as well as the difficulty encountered in preparing such polymers, limits their use. Thus, there is currently a need for secondary sulfonamide polymer matrices possessing advantageous properties (e.g. improved base stability, improved transport, improved adsorption, or improved selectivity of binding). There is also a need for improved methods for preparing secondary sulfonamide polymers.